1. Field of the Invention
The present invention relates to a method and apparatus for the discharge treatment of gases. More particularly, the present invention relates to a method of treating noxious components in off-gases from factories or the like by catalytic oxidation. The present invention also relates to a method of disinfecting the air or the surfaces of various objects with the aid of ozone. The present invention additionally relates to a discharge method and apparatus which may be employed in these methods.
2. Prior Art
A variety of methods have heretofore been proposed and practiced in order to treat waste gases in manners that best suit the properties of the target components in the gases. For example, the air that is evolved in wastewater treatment plants and which contains such noxious components as hydrogen sulfide and mercaptan is usually treated by absorption, adsorption, oxidation or masking techniques. Of these methods, the catalytic oxidation process which employs platinum-alumina catalysts is considered to be most advantageous because of its high elimination efficiency.
Gases containing organic solvents are discharged from petrochemical plants and printshops or printworks. The organic solvents are recovered by the combination of adsorption on activated carbon and regeneration of steam if the concentrations of the organic solvents are as high as a few percent. Treatment by catalytic oxidation is performed if the concentrations of the organic solvents are on the order of 10.sup.2 ppm.
It has been proposed that boiler off-gases containing sulfurous acid should be treated by catalytic oxidation with vanadium catalysts, rather than by wet adsorption with calcium carbonate, in order to recover sulfuric acid.
As illustrated above, catalytic oxidation is being used increasingly as a method for treating waste gases. The catalytic oxidation process has the advantage of permitting lower reaction temperatures to be employed than the direct oxidation method but it still needs temperatures in the range of from about 200.degree. to 400.degree. C. in order to attain satisfactory results. Not only does this require increased energy costs in heating the gas to be treated but it also cases adverse effects on the lifetime of the catalyst employed. The catalysts employed in the conventional catalytic oxidation methods generally have lifetimes ranging from 1 to 2 years.
With the recent advances of medical technology, the interflow of personnel and equipment has increased in medical facilities and this is causing an increase in the chance of cross infection in patients. Waiting rooms in hospitals are particularly vulnerable to cross infection originating from outpatients. Therefore, an increasing number of medical facilities today have introduced sanitizing air conditioners as infection preventing means. The need for a sanitary environment has also been recognized in the food industry and bioclean rooms are being increasingly adopted by manufacturers of aseptically packed longlife foods.
Practically all of the methods practiced today for disinfecting air have relied upon the use of air filters. In this system, the air to be treated is passed through filters such as to trap both dirt and microorganisms with the resulting clean germ-free air being supplied into the clean room. The disadvantages of this system are as follows: increased electric energy costs for blowers will result from the pressure drop across the filters; a need exists for the maintenance of the filters; and insufficient maintenance of the filters may lead to contaminated air ducts and allow germ-containing air to be supplied into a clean room.
Sterilization of air by ozone has been proposed as an alternative to the use of air filters (see, for example, Unexamined Published Japanese Patent Application No. 115593/77). Ozone has been known to have a sterilizing action. Ozone itself is decomposed to harmless oxygen and hence is considered to be an advantageous air sterilizer. However, the clean air obtained by sterilization with ozone has residual ozone present. Ozone is harmful to humans even if it is present in low concentrations and generally its content in working areas must not exceed 0.1 ppm. In order to meet this requirement, the residual ozone in the clean air must be degraded by some means such as the spontaneous decomposition method or the thermal decomposition method. Ozone is a chemically highly labile substance and decomposes to oxygen upon standing. However, the half life of ozone at ordinary ambient temperatures is comparatively long (ca. 16 hours) and this makes the spontaneous decomposition method unsuitable for commercial use. The thermal decomposition method requires high operating costs for heating purposes and the air must be cooled to ambient temperature after heat decomposition. It is therefore difficult to integrate the thermal decomposition method into air conditioning systems. The applicant of the present invention previously proposed a method of degrading ozone by microwave energy (see Japanese Patent Application No. 8275/1984). Although this method has eliminated the defects of the conventional techniques of ozone decomposition, it still has the disadvantages of low energy efficiency and the need for using an expensive apparatus.
The second problem encountered in applying ozone sterilization to air conditioning systems is its slow effect. The rate of sterilization by ozone depends on its concentration but usually ozone must be held in contact with air for a period no shorter than several hours. Therefore, considerable difficulty exists in applying the conventional techniques of ozone sterilization to air conditioning systems which are required to disinfect a large volume of air.
The applicant of the present invention previously developed an improved method of ozone sterilization in Japanese Patent Application No. 98432/1984 (laid open for public disclosure Nov. 30, 1985). This method is characterized by performing discharge treatment on air containing both ozone and unwanted microorganisms such that ozone decomposition is carried out simultaneously with the killing of the microorganisms.
Discharge apparatus are currently used in a variety of fields. For example, they are incorporated in ozone generators or precipitators which remove dust particles from waste gases by electrostatic precipitation. Discharge apparatus are also being used for sterilization and deodorization purposes by making use of the characteristic features of the discharge phenomenon.
The conventional discharge apparatus forms a non-uniform electric field between electrodes (which may either be two needles or one may be a needle and the other a plate) and generates a corona discharge, glow discharge, arc discharge or spark discharge by establishing appropriate discharge conditions. Whichever type of discharge is selected, the phenomenon of discharge itself is unstable and a plurality of electrode pairs must be provided in order to produce the desired electric field in an industrial-scale apparatus. If the electrodes are directly connected to the power source, it is difficult to obtain a uniform and stable discharge at all discharge gaps because the state of the fluid to be treated is not necessarily uniform at each of the discharge gaps; and the discharge gap length may differ slightly from one gap to another. In addition, the abrupt increase in current upon discharging may break some or all of the electrodes. In order to avoid these problems, the conventional discharge apparatus has a resistor incorporated in the circuit connecting an electrode to the power source (see, for example, Japanese Patent Publication No. 23147/1979). Since the resistor is provided for each electrode, an increase in the number of electrodes incorporated in the apparatus must be accompanied by a corresponding increase in the number of resistors. A further problem arises from the fact that all the power consumed by the resistors is dissipated as heat and will not contribute at all to the efficiency of discharge.